Olefins such as ethylene and propylene may be produced by the catalytic dehydrogenation of a hydrocarbon feed or the cracking of a hydrocarbon feed. The term “cracking” will be used throughout this specification to embrace both of these chemical reactions.
The cracking of hydrocarbons is an endothermic process. Accordingly, heat has to be consumed for the reaction to occur. Auto-thermal cracking is a known process for the production of olefins from a reactant mixture comprising a hydrocarbon feed and an oxygen-containing gas. An example of an auto-thermal cracking process is described in EP-A-0 332 289.
In an auto-thermal cracking process, the heat required for cracking is generated by combusting a portion of the original hydrocarbon feed. This is achieved by passing a mixture of a hydrocarbon feed and an oxygen-containing gas over catalyst capable of supporting combustion beyond the fuel rich limit of flammability. The hydrocarbon feed is partially combusted, and the heat produced by the combustion reaction is used to drive the cracking of the remainder of the feed into olefins. Optionally, a hydrogen co-feed is also burned, and the heat produced by this combustion reaction is also used to drive the cracking of the hydrocarbon.
In an auto-thermal cracking process, the time for which the reaction mixture (hydrocarbon and an oxygen-containing gas) is in contact with the catalyst (the contact time) is believed to have an impact on the olefin yield of the overall process. Olefin yield is determined by the selectivity of the process towards olefins and the extent of hydrocarbon conversion. For high olefin yields, high selectivity and high conversion are desirable. In general, the conversion of hydrocarbon increases as the contact time increases. Without wishing to be bound by any theory, it is believed that this is because there is more time available for the hydrocarbon to react. However, increasing the contact time tends to have a detrimental effect on the selectivity to olefin, as there is more time for the olefin produced to take part in further (undesirable) reactions.
An indication of contact time can be obtained by measuring the linear velocity of the feed gases upstream from the catalyst at standard temperature (273 Kelvin) and the operating pressure of the process. This measurement, known as the superficial feed velocity, is measured in centimetres per second (cm s−1). The higher the superficial feed velocity, the shorter the contact time of the feed for a given catalyst quantity and aspect ratio.
Conventional understanding thus indicates that if high superficial feed velocities are employed in an auto-thermal cracking process the hydrocarbon feed conversion and olefin yield would be significantly reduced. Indeed, it would be expected that conversion and olefin yield would be reduced to such an extent that any potential benefits associated with operation at high superficial feed velocities would be negated.
This teaching has been exemplified by prior art catalytic oxidative dehydrogenation processes. Prior art catalytic oxidative dehydrogenation processes have been operated at superficial feed velocities of up to 265 cm s−1, but, more typically, such processes are operated at superficial feed velocities of less than 180 cm s−1.
U.S. Pat. No. 5,639,929 discloses an oxidative dehydrogenation process using a fluidised bed catalyst of Pt, Rh, Ni or Pt—Au supported on α-Al2O3 or ZrO2 and total feed flow rates of 0.5 to 2.0 SLPM (standard litres per minute) corresponding to superficial feed velocities of ˜1 to ˜4.1 cm s−1 at standard temperature and operating pressure.
U.S. Pat. No. 5,905,180 discloses a catalytic oxidative dehydrogenation process wherein the total feed flow rates “ranged from 5 SLPM”, corresponding to a superficial feed velocity of ˜24 cm s−1 at standard temperature and operating pressure.
Schmidt et al (J. Catal., 191, 62–74 (2000)) describes an oxidative ethane oxidation over a Pt—Sn/α-Al2O3 catalyst at a total feed flow rate (ethane, hydrogen and oxygen reactive components, nitrogen diluent) of 4 to 16 standard litres per minute (SLPM), corresponding to a superficial feed velocity of ˜22 to ˜88 cm s−1 at standard temperature and operating pressure. A small fall in ethylene yield was reported on raising the gas flow to the higher figure.
Holmen et al, Studies in Surf. Sci. and Catal., 119, 641–646 (1998) disclose the use of Pt and Pt/Rh gauze catalysts for oxidative ethane dehydrogenation. Experiments were conducted at a total gas feed rate of 2 standard litres per minute over a Pt/Rh gauze (which corresponds to superficial velocities up to ˜265 cm s−1 at standard temperature and operating pressure). Although they report that the formation of olefins (selectivity) is favoured by short contact times, they also note that conversion was reduced at high velocities when compared with results at ˜19 cm s−1 unless more heat was applied to the reactor externally.
WO 00/14035 discloses a process for the partial oxidation of paraffinic hydrocarbons to form olefins. The process is carried out in the presence of hydrogen and the use of gas hourly space velocities of greater than 50,000 h−1 to generally less than 6,000,000 h−1 is disclosed. In one example there is disclosed the partial oxidation of ethane in the presence hydrogen and a ceramic supported Pt/Cu catalyst at gas feed rates of up to 42 standard litres per minute and at a pressure of 1.68 bara. This corresponds to superficial feed velocities up to ˜164 cm s−1 at standard temperature and operating pressure.
U.S. Pat. No. 4,940,826 discloses a catalytic oxidative dehydrogenation process with a hydrocarbon stream consisting of ethane, propane or butane or mixtures thereof over platinum supported on cordierite monolith or over a bed of platinum on alumina spheres. The total feed flow rates range from 16.0 to 55.0 standard litres per minute corresponding to superficial feed velocities of ˜45 to ˜180 cm s−1 at standard temperature and operating pressure.
U.S. Pat. No. 5,382,741 discloses a catalytic oxidative dehydrogenation process carried out at elevated pressures (10 barg) over platinum and palladium supported on a foam monolith or on a bed of alumina spheres. The hydrocarbon feeds exemplified are propane and naphtha. The total feed flow rates range from 2.1 SLPM at 1 bara to 280 SLPM at 11 bara, corresponding to superficial feed velocities of ˜44 to ˜240 cm s−1 at standard temperature and operating pressure.